1. Field of the Invention
The invention relates to a heatable glass pane, comprising two contact bus bars of different polarity, which are arranged essentially parallel to one another in their longitudinal direction and parallel to a base edge of the heatable glass pane, and a set of heating wires, which are in electrical contact with the contact bus bars.
2. Description of Related Art
Such a glass pane has been disclosed in DE 101 26 869 A1. Therein, an electrically heatable pane is disclosed, in which the mutually parallel contact bus bars are arranged at different distances from the base edge and thus one on top of the other, when viewed transversely with respect to their longitudinal direction. The heating wire is guided, starting from the upper contact bus bar, in a plurality of loops and then beyond the upper contact bus bar to the lower contact bus bar, the heating wire being electrically insulated with respect to the upper contact bus bar at the point of intersection with said upper contact bus bar. The arrangement of the two contact bus bars on the same pane edge is particularly advantageous for side windowpanes of motor vehicles, since a contact bus bar at any other edge would be visually disruptive. In addition, the base edge lies within a doorframe, which makes power supply easier, in particular if a window winding motor is already present. The disclosure also includes guiding the heating wire in the region of a slope of the side windowpane, i.e. where the pane has not yet reached its maximum height extent, in a greater number of loops (for example with five changes in direction) than in the region of the greatest pane height (for example three changes in direction). This is intended to achieve a situation in which the lengths of the laid heating wires are the same, as far as possible, in order to achieve corresponding resistance values. However, one disadvantage of the described manner of laying the heating wires is that only a very rough variation of the heating wire length is possible. When the heating wire reaches the region of the contact bus bars, the decision remains as to whether to make contact with the contact bus bars or to carry out a further two changes in direction, which then means an increase in the heating wire lengths by almost two complete heights of the pane at this point. It is thus barely possible to achieve a situation in which the heating wire lengths are actually to a large extent exactly the same. In addition, it is technically complex to guide the heating wire with insulation over the upper contact bus bar to the lower contact bus bar.
DE 296 06 071 U1 has disclosed a heatable glass pane, in which parallel-connected, electrical heating wires, which are embedded in a thermoplastic intermediate layer, are provided only in the region of the window wiper rest position. In this case, contact bus bars have been disclosed which have only relatively small dimensions on the pane and therefore also only permit a correspondingly limited number of heating wires for contact-making purposes. The heating wires are guided in each case one loop, the heating wires, which are guided in the outermost loop, being essentially longer than the innermost wires. The limited extent of the contact bus bars and thus the limited number of heating wires which can be used allows for an expedient heating field only in a lower subregion of the glass pane.